Induction accelerator for electrons



NOV. 4, 1952 BlERMAN 2,617,026

INDUCTION ACCELERATOR FOR ELECTRONS Filed Aug. 23, 1949 ADJUSTABLE POLE PIECES SOURCE OF OSCILLATIONS N FIELD COIL FIELD con.

ANODE VOLTAGE DISCHARGE TUBE HEATlNG CURRENT SU PPLY SUPPLY ELECTRON GUN ADJUSTABLE AHZ GAP Aouwmew POLE PIECES Joupcs 0F IN VEN TOR.

fi/voo E m t a4 TA 66 W 3 AGENT.

Patented Nov. 4, 1952 INDUCTION ACCELERATOR FOR ELEGTRONS Aron Bierman, Eindhoven, Netherlands, assignor to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application August 23, 1949, Serial N 0. 111,839 In the Netherlands August 27, 1948 3 Claims. 1

Induction accelerators for electrons generally comprise a substantially wholly closed iron circult for the magnetic power flux which is produced by coils carrying a low-frequency alternating current. In such devices electrons are accelerated in an annular path by means of the electric power generated in this path by the field strength of the magnetic field which is variable with time. The iron body comprising the yoke and the pole-shoes and serving for generating the magnetic flux is a considerable weight. Consequently, it has been suggested to make the device smaller, lighter and cheaper by omitting the llOll.

' In order that the required number of ampere turns may be obtained in an ironless circuit for generating the considerable magnetic field strength required to ensure energy of some magnitude in the discharge path, use is made of the high-frequency oscillation of an electric load.

In order that the electrons may describe a circular path, not only an accelerating field but also a magnetic control field is required. The lines of force of the accelerating field are circumscribed by the electron path and those of the control field are intersected thereby. It is necessary that the field strength at the electron path should be half that which would occur throughout the surface circumscribed by the path if the central fiux were evenly distributed over this surface. In a known device comprising two coils arranged on each side of the annular electron path and traversed by the electric load, the central field is too weak to satisfy this condition. In a further known device, this disadvantage is obviated by the provision in addition to the two coils arranged on each side of the discharge tube of a central coil surrounded by the annular discharge tube which is traversed by the electric load as well as the two coils arranged laterally of the tube.

When the electrons have reached their maximum velocity, that is by the moment at which the accelerating action due to the magnetic field ceases, they are required to be brought out of their path. In the last-mentioned known device comprising an ironless circuit, this is effected by means of a phase-shift between the currents traversing the central coil and the auxiliary coils. For this purpose the central coil is connected in parallel with a resistance. However, loss of energy occurs, which is a disadvantage of this arrangement.

A further means of obtaining the required contraction or expansion of the electron path is used in apparatus comprising a magnetic circuit constituted by an iron body. In this case use is made of the fact that the centre of the iron of the magnetic circuit is saturated earlier than the part in the vicinity of the electron path since the field is materially stronger at the centre.

In another known apparatus use is made of auxiliary poles to modify the magnetic field in the vicinity of an electron path, in which the magnetic circuit of this control field is constructed with a portion having a smaller crosssectional area so that, upon increase of the field during the acceleration period, saturation occurs in this portion earlier than in the central core.

In either case, when the electrons have attained their maximum velocity, the condition that the magnetic power fiux circumscribed by the electron path should be twice the field strength at the path multiplied by the surface of the circle within this path, is no longer satisfied. In the former case the electrons diverge inwardly and in the latter case outwardly.

The present invention relates to such apparatus in which the magnetic circuit fundamentally comprises so little iron that the high-frequency oscillation of an electric load is required to enable magnetic fields of sufiicient strength to be generated. In this case it has been found possible to obtain contraction of the electron path as simply as in the said constructions of induction accelerators comprising a magnetic circuit extending wholly through iron.

According to the invention, instead of utilising a central coil, use is made of an open iron core, hence a core which extends not farther than is necessary for concentrating the central flux within the zone circumscribed by the discharge tube. It has been found possible with the use of the said core to fulfill the requirement concerning the ratio between the actual fiux and that of a homogeneously imagined field and to proportion the sectional area of the core in such manner that this ratio is disturbed at the end of the acceleration period as a result of saturation -of the core.

The proper ratio between the total flux and the field strength at the electron path is adjustable in a simple manner by providing an adjustable air-gap, in which event the core may consist of two portions which are relatively slidable. The total flux decreases when the parts of the core are moved away from one another.

The presence of an air-gar) at the centre of the core located in the plane of the electron path furthermore favourably affects the configuration 3 of the magnetic control-field within the tube. In addition to the fact that the so-called flux condition must be fulfilled, owing to which the electrons describe a circular path, it is of importance that electrons diverging from this path due to some reason or other, for example impact with gas molecules, should return to the path by the action of the magnetic directional field. It is known that for this purpose the magnetic field strength in the vicinity of the electron path is required to be proportional to r", I being theradius of the path and n a figure between 0 and 1.

In the device according to the invention, this requirement is fulfilled but it appears that the value of n varies between the limits 0 and 1 upon radial displacement of the path of, the electron. This implies that the ratio between, the axial, and

the radial stability in the vicinity of the electron path is different from that at the path which the electrons have to follow, which results in the dimensions of the beam not being in conformityw-ith the choice of the ratio between the axial and the radial stability in the electron path. This may lead to a loss of electrons which, as a result of disturbing influences, diverge from the path of the beam to a greater extent than would otherwise be the case. When an air-gap is provided at the centre of the core, the stray field produced at this areamay serve to correct the configuration of the field within the tube. The air-gap may be so adjusted so that n has a constant value over a suificiently' wide range. Variation of the air-gap also varies the central flux so that th flux condition is affected. In order that, in view thereof, a new correction may be made, one or more further air-gaps may be provided to permit of controlling the value of the flux and which are located at such a distance from the plane of the electron path as to act no longer upon the configuration of the magnetic field within the tube.

The invention will be described in connection with the accompanying drawing in which:

Fig. 1 shows a schematic representation of one embodiment of an induction accelerator accord- The induction accelerator shown in Fig. 1 comprises a closed annular vessel I forming a discharge path for electrons emitted by a cathode 2 forming part of an electron gun 3. The electrons are accelerated in an orbital path in the vessel by a pair of coaxial field coils 4 and 5 which are coaxial with the axis of the discharge vessel and are excited by a generator of electrical oscillations I3.

The flux generated by the coils traverses the path of the electron stream in the discharge vessel and the ratio of total flux to field strength at the. electron path, in. order to satisfy the condition that the magnetic: field strength in the vicinity of the electron path is proportional to r-, 1' being the radius of the path and n a figure between 0 and 1, is adjustable by providing a core 5, 1 coaxial with the coils having an adjustable air-gap. The core shown in Fig. 1 is of the yokeless type and comprises two sections 6 and I which are separated by the air-gap but not joined by a yoke. By moving either or both sections of the core, th air-gap distance can be adjusted so that n has a constant value over a sufiiciently wide range.

The arrangement shown in Fig. 2 is similar to that shown in Fig. 1 except that the core comprises several. disconnected separate sections, 9, [0, II and 12 the central sections 9 and 10 being separated by an air-gap for determining the value of n. Thaadditional air-gaps afiorded by further dividing the core into a plurality of sections permits' greater control of the flux traversing the discharge path.

What I claim is:

1. An electron induction accelerator comprising a closed vessel defining an annular discharge path, means including a cathode in said vessel for producing an electron stream in said vessel, and magnetic field producing means outside said vessel comprising a set of axially aligned opposed coils disposed on opposite sides of the discharge path and coaxial with the central axis of said discharge path, means connected to said coils to excite said coils for producing a time varying magnetic field, and an axially extending yokeiess core of magnetic material disposed between said coils and extending axially through said discharge path, said core having a cross-sectional area at which saturation of the core results at the end of the acceleration period of electrons in the discharge tube.

2. An electron induction accelerator as claimed in claim 1 in which the core comprises two separate and disconnected sections separated by an air-gap in the plane passing through the electron discharge path.

3. An electron induction accelerator as claimed in claim 1 in which the core comprises a plurality of separate disconnected sections separated by air-gaps spaced at a distance from the plane of the electron discharge path at which the configuration of the magnetic field within the vessel is substantially unafiected.

ARON. BIERMAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,137,964 Goddard, May 4, 1915 2,103,303 Steenbeck Dec. 28, 1937 2,191,594 Spencer Feb. 24, 1940 2,297,305 Kerst Sept. 29, 1942 

